1. Field of Invention
The present invention relates to a tunable organic light emitting diode. More particularly, this invention is directed to a full-color tunable resonant cavity organic light emitting diode (RCOLED).
2. Description of Related Art
Visible organic light emitting diodes (OLEDs) have attracted a great deal of attention recently for applications such as color display and printing. Large area OLED arrays are a potential alternative to active matrix liquid crystal displays (AMLCDs). The advantage of OLEDs derives principally from the lack of polarizers, color filters and alignment layers which compliment the AMLCD assembly. These polarizers, color filters and alignment layers reduce the efficiency of the AMLCDs in producing emitted light.
To increase the efficiency of OLEDs, resonant cavity structures have been used to spatially and spectrally redistribute the electro-luminescence of organic materials utilized in the OLEDs. Light emission in the forward direction from a RCOLED can be increased by as much as four times over that emitted from a non-cavity OLED. Additionally, the spectral width of the RCOLED is much narrower than that of a non-cavity OLED. A narrower spectral width provides a purer color in the forward direction along the viewing axis of the OLED.
Conventionally, a red/green/blue tri-element side-by-side array of RCOLEDs has been used for color display applications. The red/green/blue tri-element side-by-side RCOLED array uses resonant cavity structures in which each element has a different cavity length to produce a specific color.
FIG. 1 shows a typical RCOLED 100. The RCOLED 100 includes a glass substrate 110, a high-reflection quarter-wave stack 120, a contact layer 130, a hole transport layer 140, an emitting medium 150, an electron transport layer 160 and an electrode 170. The high-reflection quarter-wave stack 120 is preferably formed of dielectric layers such as SiO.sub.2 /TiO.sub.2. The contact layer 130 is preferably indium tin oxide (ITO) and is electrically coupled to the electrode 170. The hole transport layer 140 is preferably formed of triphenyl diamine (TAD). The emitting medium 150 and the electron transport layer 160 are preferably formed of 8-hydroxyquinoline aluminum (Alq). The electrode 170 is preferably formed of aluminum. The resonant cavity of the RCOLED 100 is formed by the quarter-wave stack 120 and the electrode 170. In operation, in the RCOLED 100, light is emitted through the glass substrate 110. The effective resonant cavity length determines the on-axis resonant wavelength produced by the RCOLED 100.
FIG. 2 shows the electro-luminescent spectra produced by a RCOLED such as the RCOLED 100 illustrated in FIG. 1. FIG. 2 shows the electroluminescence spectra of the RCOLED measured at 0, 10, 20, 30 and 40 degrees from a line normal to the surface of the substrate 110. The light intensity at 0.degree. is greatly enhanced over the light intensity of a non-cavity OLED. However, the intensity of the produced light drops off quickly along the off-axis directions, e.g., 10, 20, 30 and 40 degrees from the normal line. Additionally, there is a spectral shift associated with the off-axis directions. These effects are problematic for display applications where a wide viewing angle is preferred.